LNG ship unloading is in many cases a critical operation that requires efficient integration with a regasification operation. Typically, when LNG is unloaded from an LNG ship to a storage tank, LNG vapors are generated from the storage tank due to volumetric displacement, heat gain during LNG transfer and in the pumping system, storage tank boiloff, and flashing due to the pressure differential between the ship and the storage tank. In most cases, the vapors need to be recovered to avoid flaring and pressure buildup in the storage tank system.
In a typical LNG receiving terminal, a portion of the vapor is returned to the LNG ship, while the remaining vapor portion is compressed by a compressor for condensation in a vapor absorber that uses the refrigeration content from the LNG sendout. Therefore, vapor compression and vapor absorption systems generally require significant energy and operator attention, and particularly during transition from normal holding operation to ship unloading operation. Alternatively, vapor control can be implemented using a reciprocating pump in which the flow rate and vapor pressure control the proportion of cryogenic liquid and vapor supplied to the pump as described in U.S. Pat. No. 6,640,556 to Ursan et al. However, such configurations are often impractical and generally fail to eliminate the need for vapor recompression in LNG receiving terminals.
Alternatively, or additionally, a turboexpander-driven compressor may be employed as described in U.S. Pat. No. 6,460,350 to Johnson et al. Here the energy requirement for vapor recompression is typically provided by expansion of a compressed gas from another source. However, where a compressed gas is not readily available from another process, generation of the compressed gas is energy intensive and uneconomical.
In other known systems, methane product vapor is compressed and condensed against an incoming LNG stream as described in published U.S. patent application to Prim with the publication number 2003/0158458. While Prim's system increases the energy efficiency as compared to other systems, various disadvantages nevertheless remain. For example, vapor handling in Prim's system is typically limited to plants in which production of a methane rich stream is desired.
In yet another system, as described in U.S. Pat. No. 6,745,576, a plurality of mixers, collectors, pumps, and compressors are used for re-liquefying boil-off gas in an LNG stream. In this system, the atmospheric boil-off vapor is compressed to a higher pressure using a vapor compressor such that the boil-off vapor can be condensed. While such a system typically provides improvements of control and mixing devices in a vapor condensation system, it nevertheless inherits most of the disadvantages of known configurations as shown in Prior Art FIG. 1.
Moreover, the composition and heating values of most imported LNG varies dramatically and will generally depend on the particular source. While LNG with heavier contents or higher heating value can be produced at lower costs at the source, they are often not suitable for the North American market. For example, natural gas for the Californian market must meet a heating value specification of 950 Btu/SCF-1150 Btu/SCF, and must meet composition limitations on its C2 and C3+ components. Especially where LNG is used as transportation fuel, the C2+ content must be further reduced to avoid high combustion temperature and reduce greenhouse emissions. Table 1 below depicts composition requirements in comparison to a typical imported LNG supply. Thus, it would also be desirable to configure an LNG receiving terminal with the capability to accommodate to varying LNG compositions.
TABLE 1COMPONENTNATURAL GAS SPEC.TYPICAL LNG SUPPLYC188% minimum87% to 94%C26% maximum3% to 7%C3-C53% maximum1% to 7%C6+0.2% maximum0.1% to 0.8%N21.4 to 4.5%Less than 0.1%Gross Heating970-11501050-1200Value, Btu/SCF
Unfortunately, most of the currently known processes and configurations for LNG ship unloading and regasification fail to address various difficulties. Among other things, many of the known processes require vapor compression and absorption that are energy inefficient. Still further all or almost all of the known processes fail to economically remove heavy hydrocarbons from LNG to meet stringent environmental standards. Thus, there is still a need to provide improved configurations and methods for gas processing in LNG unloading and regasification terminals.